Known in the art are methods of preparing polycrystalline cubic boron nitride from hexagonal boton nitride at high pressures and temperatures in the presence of catalysts such as alkaline, alkaline-earth metals, tin, lead, antimony, as well as nitrides of the above-cited metals (K. Svenson, "High-Pressure Physics", Moscow, 1963, in Russian). In additiona, as catalysts use can be made of water of compounds liberating water in the process of synthesis.
According to the known methods, the catalysts are introduced into the charge either in the form of separate chemical compounds or as compounds which can decompose to said catalysts under the action of high pressures and temperatures. Known methods make it possible to prepare polycrystalline cubic boron nitride containing the products of interaction of the catalyst with boron nitride, which causes gradual weakening of the boron nitride crystalline structure.
Also known in the art is a method of preparing boron nitride of cubic structure residing in that hexagonal boron nitride is subjected to the action of pressure of 40-90 kbar and temperature of 1200.degree.-2400.degree. C. in the presence of a catalyst. As a catalyst use is made of alkaline, alkaline-earth metals, nitrides thereof, as well as antimony, tin, or lead.
In the process use can also be made of a mixture of two and more catalysts and of alloys containing catalysts and non-catalyst metals. The prepared boron nitride contains the products of interaction thereof with a catalyst, for instance, Li.sub.3 BN.sub.2, Mg.sub.3 BN.sub.3.
The method is disadvantageous in that the prepared boron nitride contains the products of interaction thereof with a catalyst. These compounds have a low melting point, high reactivity, and decompose rapidly in humid air.
Being in the bulk of polycrystalline formations of boron nitride, the above-cited compounds in the process of treating hardened steels and alloys at high temperatures favour destruction of cutting edges of the tools manufactured from boron nitride.